1. Field of the invention
The present invention relates to a high voltage positive electrode active material for lithium battery, more particularly, to a positive electrode active material based on lithium manganese spinel oxide wherein manganese is partially substituted with nickel
2. Description of the Related Art
As positive electrode active material for lithium battery, manganese dioxide for primary battery and vanadium oxide (V2O5), lithium cobalt oxide (LiCoO2 ), etc. for secondary battery have already been put in practical applications, while many other materials have been proposed including lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4). Among these materials, lithium manganese oxide is regarded as promising because of the low cost of the production, and nontoxicity of manganese. Representative among lithium manganese oxides is the spinel oxide (LiMn2O4) Which has a three dimensional structure. Charge and discharge reactions of a lithium secondary battery wherein manganese spinel oxide is used as the positive electrode active material take place in the following two stages.
Li1xe2x88x92xMn2O4.+xLi++xexe2x88x92xe2x86x92LiMn2O4xe2x80x83xe2x80x83(1)
LiMn2O4+Li++exe2x88x92xe2x86x92Li2Mn2O4xe2x80x83xe2x80x83(2)
The discharge process represented by equation (1) takes place at a potential of about 4V with respect to a lithium reference electrode (Li/Li+) with the crystal in this process having cubic structure, while the discharge reaction represented by equation (2) takes place at a potential of about 3V or lower with the crystal structure changing from cubic structure to tetragonal structure. Theoretical discharge capacity density is 154 mAh/g in both cases of equations (1) and (2).
However, as reported in J. Electrochem. Soc., 137,769, (1990), etc., lattice constant changes resulting in a change in the unit cell volume in the discharge process of (0xe2x89xa6xxe2x89xa61), despite the crystal structure remains in cubic structure. According to Mate., Res, Bull., 18,461 (1983) and Solid State Ionics 69,59 (1994), when the value of x approaches 1 in the discharge process of LixMn2O4 (0xe2x89xa6xxe2x89xa61) and further shifting to LixMn2O4 (1xe2x89xa6xxe2x89xa62), the crystal structure changes from cubic structure to tetragonal structure due to Jahn-Teller effect. At this time, a great change in the unit cell volume occurs because the value of crystal lattice constant ratio c/a increases by 16%. Such a change in volume causes the electron collecting performance of the electrode to decrease, resulting in the decrease in the capacity. Because the volume change due to Jahn-Teller effect is particularly substantial, it has been common to use the battery in the region of 4V corresponding to equation (1) or in the region of 3V corresponding to equation (2). The decrease in the capacity with the process of charge and discharge processes in a battery employing lithium manganese spinel oxide as the positive electrode active material is also attributed to the dissolution of manganese ions included in the crystal into the electrolyte, as reported in J. Power Sources, 43-44, 223 (1993) and J. Power Sources, 52, 185 (1994). That is, average valence of manganese is over 3.5 during the process of equation (1), but decreases below 3.5 during the process of equation (2) during which the amount of trivalent manganese ion increases. With the presence of manganese ions having a valence of 3, a disproportionate reaction of equation (3) occurs where the produced divalent manganese ions are partially dissolved in the electrolyte of Lithium thereby, resulting in the decomposition of active material and the loss of the reversibility of the electrode.
2Mn3+(solid phase)xe2x86x92Mn4+(solid phase)+Mn2+(dissolved)xe2x80x83xe2x80x83(3)
The dissolution of manganese can also be observed through change of color of the organic electrolyte from colorless to light purplish red. Thus keeping the valence of manganese as high as possible is effective in preventing the decrease in capacity.
The Journal of Electrochemical Society, 138, (10), 2859 (1991) discloses an attempt of substituting part of manganese with another metal. It is reported that, in case nickel is employed as the substituting metal, when discharging in a range from 45V to 2.0V while changing the value of y in chemical formula LixMn2xe2x88x92yNiyO4, three potential plateaus are observed at 3.9V, 2.8V and 2.2V and the discharge capacity shows greater decrease at both 3.9V plateau and 2.8V plateau as the value of y increased. This doped spinel is prepared in solid phase reaction by means of firing, while using Li2CO3, MnO2 and the oxide of the partially substituting metal as the starting materials.
EP0712173A1 discloses that the use of a lithium manganese composite oxidedefect type spinel structure provides monotonous variation of the potential during charging and discharging process. Further, W096/10538 discloses that ternary lithium mixed oxides of general formula LiyMeMn2xe2x88x92xO4 having a spinel type in which Me means various metals, are suitable as a cathode material for a lithium secondary battery. However, both of these reference relate to conventional 4V class lithium batteries.
In the Japanese Patent Application Laid-Open No3-285262, it is reported that good cycle characteristic is obtained from an electrode fabricated from a positive electrode active material represented by general formula Li1+yMn2xe2x88x92zAzO4 (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa60.5, and A is at least one element selected from the group consisting of Ti, V, Cr, Mo, Ni and Fe), an electricity conducting agent and a binder. A battery employing Li1.1Mn1.8Co0.2O4 as the positive electrode active material and metallic sodium as the negative electrode active material, for example, shows discharge characteristic ranging from 4.1V to 3.7V.
The Japanese Patent Application Laid-Open No.63-274059 discloses that a battery represented by a general formula LiMn2O4 employing a positive electrode active material, which shows X-ray diffraction peak of half-power width from 1.1 to 2.1 when irradiated with Fe(ka) line at a diffraction angle 46.1xc2x0 and a negative electrode active material of metallic lithium has good discharge characteristic with discharge voltage being around 2.5V with a resistance of 1Kxcexa9. The positive electrode active material is manufactured through solid phase reaction with heat treatment being applied to lithium carbonate and manganese dioxide in air at a temperature from 400 to 520xc2x0 C.
The Japanese Patent Application Laid-Open No.4-87268 discloses that a battery represented by a general formula LixMn2xe2x88x92yFeyO4 (0 less than x0 less than y less than 2) employing a manganese iron-lithium compound oxide of spinel structure op similar spinel structure as the positive electrode active material and a laminated plate of aluminum and metallic lithium as the negative electrode active material shows discharge characteristic having discharge Capacity which increases When the operating voltage is in high-voltage region of 3V and 5V higher, while the discharge takes place in two stages. It is described that the active material is preferably manufactured in solid reaction by firing a mixture of oxides, hydroxides, carbonates, nitrates, etc. of Mn, Fe and Li in specified pr-proportion in an atmosphere Of air op oxygen at a . temperature above 450xc2x0 C., preferably from 600 to 1000xc2x0 C.
In the prior art, as described above, there has been such problems that partial substitution of manganese in a spinel lithium manganese oxide with another metal exhibit two plateaus or 4V and 3V during the charge and discharge processes with the crystal structure changes as charge and discharge processes are repeated, resulting in the expansion and contraction of the unit cell volume and a decrease in the capacity.
An object of the invention is to provide a positive electrode active material for very high voltage lithium battery where charge and discharge reactions proceed in one phase reaction and a monotonous variation of the potential can be obtained,
Another object of the invention is to provide a positive electrode active material for high voltage lithium battery wherein the crystal structure does not change with charge and discharge reactions, the unit cell volume experiences less change and the capacity experiences less decrease as the charge and discharge operations are repeated, and a method for manufacturing the same.
The positive electrode active material for lithium battery of the invention. represented by a general formula LixMn2xe2x88x92yMyO4 (M: a 2-valence metal selected from Ni, Co, Fe, Mg and Zn with 0.45xe2x89xa6yxe2x89xa60.60, 0xe2x89xa6xxe2x89xa61) having cubic spinel structure of lattice constant within 8.190 angstrom. In the case solid phase reaction process is employed, the positive electrode active material is manufactured by repeating the process of firing lithium carbonate and nickel nitrate in a temperature range from 750 to 850xc2x0 C. while applying pressure-treatment in the course . In case the sol-gel process is employed, one of inorganic Salt, hydroxide and organic acid salt of lithium or a mixture of these for Li, one of inorganic salt and organic acid salt of manganese or a mixture of these for Mn, and one of inorganic salt and organic acid salt of the selected metal or a mixture of these for M, are used as the starting materials for the synthesis. Gelling process to obtain gelatinous material by adding ammonia water to the solutions of these starting materials or alcohol or water and process to fire the gelatinous thus obtained, was carried out. to synthesize a compound LixMn2xe2x88x92yMyO4 (0xe2x89xa6xxe2x89xa61) with 0.45xe2x89xa6yxe2x89xa60.60
The above and further objects and features of the invention more fully be apparent from the following detailed description with accompanying drawings.